1. Field of the Invention
The present invention relates to a novel apparatus for feeding a low vapor-pressure, liquid or solid, material into a reaction furnace for use in a CVD (Chemical Vapor Deposition) method, in which thin solid films grow on a crystalline substrate as the result of thermochemical vapor-phase reactions.
2. Description of the Prior Art
In the CVD method, in order to form thin solid films on a crystalline substrate, a growth material such as Si organic substance, tetrachlorosilane, and ethyl silicate, is deposited on the surface of the substrate in a form of oxide as a result of heat decomposition in an environment of 600.degree.-800.degree. C.
FIG. 4 shows one apparatus having been conventionally used in the CVD method, in which a low vapor-pressure liquid material is vaporized. In FIG. 4, the low vapor-pressure material 20 is accommodated in an ampoule 11. The ampoule 11 is surrounded by a thermostatic tank 12, in which the low vapor-pressure material 20 can be heated up to a certain temperature at which the low vapor-pressure material 20 vaporizes easily. Inert gas 13 is fed into the ampoule 11 through a flow control unit 14. This flow control unit 14 controls a flow amount of the inert gas 13 supplied therethrough. The inert gas 13 introduced into the ampoule 11 bubbles the low vapor-pressure material 20 having been heated by the thermostatic tank 12. Thus, vaporized low vapor-pressure material 20 and inert gas 13 are conveyed together into a vacuum chamber 17 through a gas feeding pipe 16. This gas feeding pipe 16 is heated, by a surrounding heater 15, up to a temperature higher than that of the thermostatic tank 12.
However, such a conventional apparatus has a disadvantage in controlling an amount of the low vapor-pressure material 20 to be vaporized. Namely, in order to stabilize the supply amount of the vaporized low vapor-pressure material 20, this conventional apparatus forces us to precisely control the temperature and pressure of the ampoule 11. Controlling both the temperature and pressure of the ampoule 11 precisely is not so easy.
Another disadvantage of this conventional apparatus will reside in its structural deficiency because this conventional apparatus is incapable of directly controlling the flow amount of the vaporized low vapor-pressure material. This disadvantage will become more apparent in such a case that the apparatus configuration is changed or modified for some reasons. For example, if sizes of the respective apparatus component are changed, the low vapor-pressure material will correspondingly cause a change in its flow amount. This change results in an error in the control of feeding amount of the low vapor-pressure material.
In order to solve this problem, it is desirable to use a flow control device capable of precisely controlling the flow amount of the low vapor-pressure material. Endurance temperatures of present days flow control devices are, however, limited within a relatively low temperature zone of approximately 150.degree. C. Hence, no flow control device could control a low vapor-pressure material, if a vapor pressure was not sufficiently obtained for the flow control in an environment below 150.degree. C.
As is explained in the foregoing description, the conventional apparatus could not stably control a feeding amount of a low vapor-pressure material in the CVD method because no control means directly controlling the feeding amount of the low vapor-pressure material was available in the past.